1. Field of the Invention
The present invention relates to a process and an apparatus for recovering ammonia. More particularly, the present invention pertains to a process and an apparatus for efficiently recovering ammonia from a large amount of a gas containing ammonia with a high concentration.
2. Description of the Related Arts
Ammonia is widely used not only as a raw material for chemical industries but also for the formation of a nitride film in the production of ornaments, carbide tools and semiconductors. After being used, the remaining ammonia is converted into a low-value substance or a valueless substance, which is subjected to an exhaust gas treatment, or exhausted in part into the atmosphere without being treated. In addition a large amount of useful ammonia is exhausted on occasion depending upon the step in which ammonia is used. For example, in the production of a compound semiconductor such as a gallium nitride film, most of ammonia is exhausted as such from a compound semiconductor manufacturing unit without causing any reaction at a high concentration. Such being the case, a great expense is required for a harm-removing treatment of ammonia. Thus the recovery of ammonia is eagerly desired from the viewpoints of both the environmental science and the effective utilization thereof as a valuable resource. In such circumstances, the present invention provides a process and an apparatus for efficiently recovering ammonia.
An ammonia-containing exhaust gas generated after the use of ammonia has heretofore been seldom recovered as such in usable form of ammonia, since ammonia is a relatively inexpensive compound. Instead, the ammonia-containing exhaust gas has been subjected in many cases to exhaust gas treatment by a method in which ammonia is removed, ammonia is made harmless or the like so as to discard said exhaust gas.
There have been known as a process for treating ammonia-containing exhaust gas, a combustion treatment process, a wet absorption process, a dry adsorption process, a decomposition treatment process, a combinational process of decomposition and dry adsorption and the like. However, the above-mentioned processes involve such problems as described hereunder.
The combustion treatment process suffers from the defect that a fuel such as propane is necessary for combustion treatment, the applicable range of a combustion unit is narrow for load variation, and nitrogen oxides are by-produced accompanying the combustion of ammonia. The wet absorption process in which an acidic aqueous solution is employed suffers from the disadvantage that the by-produced ammonium salt is difficult to dispose of. The dry adsorption process in which ammonia is made harmless by chemical adsorption suffers from the drawback that the use of an expensive adsorbent causes a high treatment cost in the case of treating a large amount of ammonia-containing exhaust gas.
On the other hand, the decomposition treatment process in which ammonia is decomposed into nitrogen and hydrogen by bringing ammonia into contact with an ammonia decomposition catalyst under heating can not completely decompose ammonia, since a part thereof remains undecomposed on the basis of chemical equilibrium. There is also known a combinational process of decomposition and dry adsorption in which ammonia is decomposed into nitrogen and hydrogen by bringing ammonia into contact with an ammonia decomposition catalyst under heating, followed by cleaning of the undecomposed ammonia by the use of a dry adsorbent. Nevertheless, said process suffers from the shortcoming that the use of an expensive adsorbent causes a high treatment cost in the case of treating a large amount of ammonia-containing exhaust gas.
As described hereinbefore, any of the above-mentioned processes for treating ammonia as the exhaust gas is each defective, and besides is not intended to recover ammonia as a useful substance. That is to say, the basic problem remains unsolved in that a useful substance as a raw material for chemical industries or semiconductor manufacturing is converted unfavorably into a valueless or low-value substance at a considerable cost.
On one hand, there is employed a large amount of highly pure ammonia in the production of a nitride film semiconductor and the like. In this case, however, most of ammonia is exhausted as such without causing any reaction at a high concentration and further in a large amount, and therefore it is disadvantageous to apply any of the foregoing processes to the treatment of the ammonia-containing exhaust gas.
An efficient recovery of the ammonia thus used, if made possible, can favorably contribute to not only the effective utilization of a resource but also environmental preservation. However, there is not yet proposed a process for efficiently recovering the ammonia.